In conventional synchronous signal sampling methods, high frequency repetitive waveforms are sampled at sampling points having a known phase relative to the repetitive waveform, and the phase of the sampling points is varied relatively slowly to reconstruct a signal which is a lower frequency analog of the high frequency repetitive waveform.
Such sampling is commonly performed using an electronic shutter. However, electro-optic sampling techniques have been developed for very high frequency waveforms which cannot be sampled readily using an electronic shutter. In these electro-optic sampling techniques, a series of very short optical pulses having a known polarization are passed through a semiconductor device in the vicinity of a conductor carrying the very high frequency waveform to be sampled. Electric fields due to the signal carried in the conductor affect the polarization of the optical pulses as they pass through the semiconductor, and the resulting changes in polarization are detected by a polarization-sensitive optical detector. As the optical pulses are affected by the electric fields only for the length of each optical pulse and the time it takes each optical pulse to pass through the field-affected region of the semiconductor device (both of the order of picoseconds), the time-averaged optical signal is representative of the waveform sample, and the polarization-sensitive detectors do not need to be particularly fast.
The sample signals obtained in this manner are quite small, so noise can be a problem. According to a conventional noise reduction technique, the high frequency waveform to be sampled can be modulated at a known frequency, the modulated signal can be sampled to generate a sample signal, and the amplitude of a component of the sample signal at the known modulation frequency can be detected to reduce out of band noise.
Unfortunately, such modulation may perturb the operation of the semiconductor device. This is particularly true for digital semiconductor devices which may change state as a result of the modulation of the high frequency waveform, so that the reconstructed waveform is not representative of the high frequency waveform which would result if modulation were not applied.
According to another noise reduction technique, the phase of the sampling point can be modulated at a frequency higher than the predicted noise bandwidth and the reconstructed signal can be passed through a high pass filter to reduce low frequency noise. The results of several scans can also be averaged for further noise reduction. Unfortunately, sophisticated electronics are required to cope with the high scan frequency.
According to yet another noise reduction technique, the phase of the sampling point can be dithered at a high frequency, and the reconstructed signal can be passed through a narrow bandpass filter centered on the dither frequency. Such dithered sampling gives a representation of the derivative of the sampled waveform which can be integrated to get a representation of the high speed waveform. Unfortunately, the integration step adds some noise back into the reconstructed waveform.
According to still another noise reduction technique, the polarization of the optical pulses can be modulated to modulate the sensitivity of the optical pulse polarization to the fields generated by the signal being sampled. Unfortunately, polarization modulation requires sophisticated electro-optical devices, and is sensitive to unintentional bias and drift.